I have played various XBox360 games for hours on end with no negitive effect on my 42in Plasma.
If you are planning to pause the game, turn off the TV.
The only issues I have with ghost images/burn-in is with the lil station identifiers on the bottom of some channels.
My kids love noggin but the channel identifier is a solid color and will burn in.
Alof of the channels are translucent or opaque, so when the image changes behind it, it changes the color of the station thing....
Other than that, using the 4:3 aspect with the black bars on the side can burn in. I reccomend always using a zoom\stretch.
On my TV (Dell 4201) there is a setting for plasma conditioning.
It does a great job on removing ghosting, but will do nothing for Burn-In.
Hopefully they will soon produce an LCD with the picture quality of a plasma.